Low softening point glasses are particularly useful in silver filled (or silver glass) die attach adhesives. Such adhesives, are used to attach semiconductor chips (dies) to substrates and are typically applied by heating the system beyond the softening point of the glass. The bond forms during cooling.
Because many semiconductor devices, such as silicon integrated circuits or gold sputtered devices are temperature sensitive, it is desirable to keep the peak temperature experienced by the system to a mininum. Accordingly, a number of low softening point glasses have been investigated and a series of adhesives based on "lead/borates" (PbO/B.sub.2 O.sub.3 /SiO.sub.2 or PbO/B.sub.2 O.sub.3 /Bi.sub.2 O.sub.3) have been utilized. These adhesives have softening points in the region of 375.degree. C. and glass transition temperatures (T.sub.g) in the region of 325.degree. C., and are ordinarily processed as silver glass die attach adhesives at 425.degree.-450.degree. C. (approximately 100.degree.-125.degree. C. above the respective glass transition temperatures)
While glass systems with lower softening points are known, most are not useful in die attach adhesives due to low crystallization temperatures and poor moisture resistance; further, these glasses often contain compounds such as alkali metals and fluorides, which can be detrimental to semiconductor devices, thus limiting their utility in electronic applications. For an adhesive to function, the glass must readily reform upon cooling; recrystallization generally weakens the ultimate bond strength. Thus, glasses useful in die attach adhesives must, in addition to having a low softening point, be resistant to recrystallization in the operating temperature range. Since glasses with lower softening points tend to crystallize at lower temperatures, this is a unique combination of properties.
Accordingly, it is an object of this invention to present a series of glasses suitable for use in die attach adhesives which combine the properties of a low softening point with a high resistance to recrystallization, good moisture resistance, and the absence of compounds detrimental to semi-conductor devices.